One of the major requirements put upon steel cords is full penetration of the elastomer such as rubber. This means that rubber must be able to penetrate into the cord between the composing elements and fill all possible interstices in order to reduce fretting and tensions between the elements and to avoid moisture from travelling along the cord, which would cause a lot of corrosion and which would considerably reduce the life of the cord and the rubber product.
The prior art has already provided some solutions which result in cords having rubber penetration. In patent documents U.S. Pat. No. 4,258,543 and U.S. Pat. No. 4,399,853 the solution consists of mechanically preforming the composing steel filaments of the steel cord beyond their elastical limit in such a way that the resulting steel cord takes an open form: due to the plastical deformation of the steel filaments such steel cords have `macro-gaps` along their length which ensure rubber to penetrate into the cord. Such steel cords, however, have also an important disadvantage: in order to ensure a complete rubber penetration a large degree of plastical deformation and hence a large degree of openness is required. This leads to steel cords having too great a cord diameter and having too high a part load elongation (PLE). This may cause constructional instabilities in the twisted cord. When embedded in the breaker layer of a tire, such steel cords can have a bad influence on the steering properties and on the durability of the tire.
Prior art document EP-A-0 462 716 has provided a solution to the above problem of constructional instability. It discloses a steel cord having steel filaments which have been deformed in such a way that they take a helicoidal form with a helicoidal pitch which is smaller than the pitch of the steel cord and with a helicoidal diameter which is somewhat greater than the filament diameter. Due to the fact that the helicoidal pitch of this helicoidally deformed filament is smaller than the twist pitch of the steel cord more than one `micro-gap` has been created per twist pitch. The term micro-gap is here used in order to make a distinction between the above-mentioned term macro-gaps. Micro-gaps are smaller in size but are more in quantity than the above macro-gaps. The sizes of micro-gaps are substantially smaller than the twist pitch of the steel cord. Due to these micro-gaps rubber is still able to penetrate completely into the steel cord without such steel cord having the disadvantage of a cord diameter which is too great and a part load elongation which is too great. The helicoidal filaments, however, are obtained by means of externally driven deformation pins which must rotate at a speed which is higher than twice the rotation speed of a double-twister which twists the steel cord. This is a rather energy-consuming and expensive way of manufacturing.
Yet another prior art document U.S. Pat. No. 5 ,020, 312 discloses another way of deforming some or all of the steel filaments composing the steel cord in order to obtain a steel cord with full rubber penetration. Some or all of the steel filaments pass between the tooth surfaces of a pair of gear-like elements so that they obtain a zig-zagging shape. The pitches between two teeth of these gear-like elements can be chosen so that here again more than one micro-gap between the steel filaments can be obtained per twist pitch of the steel cord. As a consequence, full rubber penetration is again guaranteed without having the disadvantage of a relatively great part load elongation. The two gear-like elements are not driven by means of an external force but are driven by the steel filaments themselves. So this process is not energy-consuming and is not expensive. This embodiment, however, has the disadvantage that the gear-like elements may roll the filaments to some extent, which may either cause damage to the steel filaments, and especially to the thin coating layer (conveniently brass or zinc with a thickness which is much less than 1 micrometer), or may lead to a lot of wear on the gear-like elements themselves, or both. Damage caused to coated steel filaments can considerably decrease the fatigue resistance and the adhesion strength of these steel filaments.